1. Field of the Invention
The subject invention relates to a method for making preforms for use in compression molding optical articles and a method and apparatus for making optical articles by compression molding.
2. Description of the Prior Art
Prior art compression molding techniques are known in the art. Such techniques can include the use of a preform. See for example U.S. Pat. No. 5,630,967 (Greshes). While such techniques are superior to conventional techniques of grinding and polishing, they suffer from disadvantages. The preforms must be kept scrupulously clean prior to use. In addition, some optical materials such as polycarbonate are hyrgoscopic. Preforms of such materials will emit water bubbles. A polycarbonate preform must be kept in an oven to remove the moisture and prevent the formation of bubbles in the finished lens. This requires having sufficient preforms of various types stored in order to meet the next day""s requirements. This can cause additional problems since polycarbonate preforms can discolor due to heat.
Extrusion compression molding processes and apparatus have been proposed. See U.S. Pat. No. 6,015,512 where a polymer strand is delivered from an extruder to a mold. However, the lenses of such process can include entrapped air bubbles. Compression, injection molding processes are also known. See U.S. Pat. No. 4,836,960 (Spector).
In compression molding techniques, a preform is often used. According to the invention, dirt, entrapped air and moisture free preform is provided for use in compression molding, particularly for compression molding of optical articles, such as ophthalmic lenses or other optical lenses. According to the invention, a method for making a preform from thermoplastic material for use in compression moldings is provided. Convex and concave mold halves are placed in a press. The molds are heated to a temperature above room temperature preferably to about the transition temperature of the thermoplastic material. A thermoplastic material is heated in a reservoir to form a fluid. The thermoplastic material is desirably a thermoplastic resin material, polymethly/methacrylate, polycarbonates, polystyrene, cellulose acetate, acrylic copolymers, thermoplastic polyesters, styrene acrylonitrile (SAN), and/or mixtures thereof. A desirable thermoplastic material for use in the preform is an amorphis cyclolofin copolymer made from monomers of norborene followed hydrogenation for example Zeonor 1020 and 1060 supplied by Zeon Chemical, Louisville, Ky. (hereinafter norborene copolymer).
A heated reservoir desirably a thermoplastic extruder is provided. The heated reservoir has a fluid outlet which is located in close proximity to the concave mold. A thermoplastic material, for example, polycarbonate pellets is fluidized in the heated reservoir. A predetermined amount of a fluid thermoplastic material is dispensed from the reservoir outlet onto the edge of the heated concave mold prior to the fluid solidifying so that the fluid makes point or line contact as it first contacts the concave molds. The fluid flows down the sloping wall of the concave mold into the concave mold and forms a blob of material having a greater thickness at the center of the concave mold than at the periphery of the mold. The thermoplastic blob is then allowed to cool below its melting temperature. A monolithic preform mass is formed having a skin and having a flat to slightly convex surface which will make point or line contact with the top convex mold. The formed preform is clean since it has been produced from clean thermoplastic material that has been melted in the reservoir. In addition, there are substantially no entrapped gas bubbles either from air or from water vapor included in the preform. Such entrapped gases or dirt can result in a defect in the finished lens. Desirably, the reservoir outlet is located as close to the surface of the concave mold as reasonably practical.
In another aspect of the invention, an improved method for rapid on-site manufacture of optical lenses in a few minutes or less is provided. According to the invention, a thermoplastic material is made into an optical lens which is substantially free of dirt, entrapped bubbles, either of air or absorbed moisture or absorbed gases. A convex and concave lens mold are placed in a compression molding press. Desirably, a plurality of presses and associated molds are used. The mold halves are preheated to a temperature above room temperature, preferably to about the transition temperature of the thermoplastic material. A thermoplastic material is heated to form a fluid.
A predetermined amount of the fluid thermoplastic material which has been fluidized in a heated reservoir, is dispensed into the concave mold through an outlet in the reservoir. The outlet is located in close proximity to the concave mold desirably as close as is reasonably feasible to allow the fluid thermoplastic to flow into the mold without building up around the outlet. The fluid thermoplastic material hits the edge of the concave mold prior to the fluid solidifying so that the fluid makes point or line contact as it first contacts the concave mold and flows towards the center of the concave mold to form a blob of thermoplastic material having a greater thickness at the center of the concave mold and a smaller diameter than the resulting molded article. The blob of thermoplastic material is then allowed to cool below its melting temperature at which time it forms a solid skin and forms a monolithic preform mass having a flat to slightly convex surface which will make point or line contact with the convex mold as it is brought down by the press to preclude an entrapment of air and the formation of air bubbles in finished lens. The press is closed and the lens molds are pressed toward one another and against the preform mass so as to mash down the preform mass to form a lens.
It is an object of the invention to form a preform in a compression mold for the manufacture of optical lenses which has reduced amounts of entrapped gases.
It is an object of the invention to form finished optical lenses having reduced incidents of entrapped gas bubbles in the finished lenses.
It is an object of the invention to provide an improved method of manufacturing lenses by compression molding a thermoplastic material.
It is an object of the invention to provide an optical lens with improved strength.
It is an object of the invention to proved as an optical lens with decreased strain.
Other and further objects will become apparent from the specific drawings and claims.
The preferred embodiment of the present invention is illustrated in the drawings and examples. However, it should be expressly, understood that the present invention should not be limited solely to the illustrative embodiment.